1. Technical Field
The present invention relates in general to magnetic sensors for reading information signals stored on magnetic media. In particular, the present invention relates to magnetoresistive heads which record and read data to and from magnetic media. More particularly, the present invention relates to dual stripe magnetoresistive heads. Still more particularly, the present invention relates to techniques for detecting common mode disturbances in magnetic media utilizing dual stripe magnetoresistive heads.
2. Description of the Related Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Magnetic disk drive units reliably store user information in the form of digital data. Inside the disk drive, the digital data serves to modulate current in a read/write head coil in order to write a sequence of corresponding magnetic flux transitions onto the surface of a magnetic medium in concentric, radially spaced tracks at a predetermined baud rate.
Transducer heads driven in a path toward and away from the disk drive axis write and read data to and from the disks. A slider supports one or more magnetic heads. As the disk is brought up to operating speed, an air bearing is generated which moves each slider and hence the heads away from the recording surface toward a preselected flying height. Achievement of a higher data density on magnetic disks has imposed increasingly narrow transducer gaps. When reading recorded data, the read/write head passes over the magnetic medium (i.e., disk)and transduces the magnetic transitions into pulses in an analog read signal that alternate in polarity. These pulses are then decoded by read channel circuitry to produce an estimated digital sequence that may contain errors caused by channel noise that obfuscate the read signal. To compensate for these errors, an error detection and correction (EDAC) system can be utilized to implement an error correction code (ECC) in order to detect and correct the errors to reproduce the originally recorded user data before passing it on to a host computer. However, such EDAC systems, by themselves, cannot accurately detect common mode disturbances in the magnetic media.
Magnetoresistive (MR) heads are well known in the art and are constructed and manufactured integral with disk drives. A magnetoresistive head comprises a magnetoresistive stripe element that measures the change in the magnetic flux directly. The resistance of the magnetoresistive stripe is inversely proportional to the strength of the magnetic flux. The resistance of the magnetoresistive stripe increases as it approaches a magnetic flux transition. When a constant current is passed through the magnetoresistive stripe, the voltage measured across it represents an analog read signal and corresponding polarity alternating pulses. Typical magnetoresistive heads thus exhibit changes in resistance in the presence of a changing magnetic field. This resistance change is transformed into a voltage signal by passing a constant current through the magnetoresistive element. The value of DC voltage, for a given head, is the product of the constant bias current and the total resistance between the head lead terminals. The temperature coefficient of resistivity of the magnetoresistive material is approximately 0.02%/degree C.
Dual magnetoresistive heads are well known in the art. Typical dual magnetoresistive heads contain good common mode rejection. However, during the manufacturing of a disk drive in which dual magnetoresistive heads are to be implemented, it is advantageous to easily detect common mode disturbances (e.g., thermal asperities) ahead of time so that such disturbances can be either screened out or utilized to gauge the cleanliness of the manufacturing processes. The same problem can also exist in a completed disk drive. When new disturbances occur, it is useful to have knowledge of their characteristics or signatures, such as thermal asperities, for invoking proper channel actions to recover the data.
Thermal asperities can locally increase the stripe temperature by more than 100 C degrees. The cause of this temperature rise is a mechanical collision of a portion of the head containing the magnetoresistive stripe with a protrusion on the disk surface. Since the change in resistance, as a function of the magnetic field due to read signal in the media, is less than 1% of the total magnetoresistive stripe resistance, the signal step that is added to the read signal when a thermal asperity is encountered can be greater than twice the base-to-peak read signal. An increase in the temperature of the stripe of 100 C degrees can cause a resistance change and a voltage change of 2%.
When the protrusion on the disk is persistent and the head continues to strike it each revolution, then the data that is being modulated by the resultant thermal induced signal transient becomes unreadable. A thermal asperity is essentially a transient in the read signal that appears when a magnetoresistive read head physically strikes an asperity on the surface of the disk, which can significantly increase the temperature of the magnetoresistive stripe element. Because the resistivity of the magnetoresistive stripe increases with temperature, a thermal asperity can cause significant transient in the analog read signal that decays exponentially.
Known arrangements for minimizing the effect of thermal asperities on the read data utilize a separate circuit asperity reduction circuit (ARC) module for additive disturbance transient suppression for data channels. Disadvantages of the known arrangements include the hardware required and the corresponding electronics cost and the required error burst length for a given thermal transient amplitude. The relatively long error site limits its applications. For example, because much more redundancy in the error correcting code or compensation (ECC) is required than is tolerable for small fix-blocked formatted drives.
Those skilled in the art will appreciate that the detection of thermal asperities and common mode disturbances is necessary since such disturbances can affect the head disk interface and the presence of too many thermal asperities on a disk surface may lead to a head disk drive "crash." Further, the knowledge that a particular defect is caused by a thermal asperity may permit special error code correction techniques to be invoked to permit recovery of data stored within the drive. Based on the foregoing, it can be appreciated that what is needed to alleviate the aforementioned drawbacks is a method and system which takes advantage of existing electrical components to detect thermal asperities and other common mode disturbances during the manufacture of a disk drive or in a subsequently completed disk drive. The invention described herein takes advantage of existing hardware and signal paths to efficiently and readily detect common mode disturbances.